Speaker
Description
We show that beam fueling of a low recycling discharge with low energy neutral beams is feasible with modest (~ 5%) lithium impurity concentrations. Low recycling discharges (R ~ 0.5) on LTX-beta have been documented with a near flat electron temperature profile. Edge temperature measurements in the SOL indicate a hot and sparse SOL with collisionality < 0.1 and values as low as 0.01. Low gradients in electron and ion temperature profiles are predicted to suppress temperature gradient driven instabilities and enhance confinement. High confinement discharges on LTX-beta can reach a H-98 of close to 2 in a limited plasma without H-mode, in the complete absence of a pressure pedestal. Fueling a low recycling discharge while maintaining a hot edge is non-trivial and will likely only be tractable either with neutral beams or with pellet injection from the high field side. Gas puffing will lead to a collapse of the hot edge. LTX-beta was recently vented to increase the tangency radius of the neutral beam. Fast ion orbit modeling indicated that injecting the beam at larger tangency radius reduces the first orbit losses and improves beam coupling. LTX-beta now shows beam fueling. Charge exchange cross section of beam hydrogen neutrals on Li ions can be 10-15 times the charge exchange cross section of beam hydrogen neutrals on ionized hydrogen from the plasma. Therefore, the effect has a functional dependence on the core lithium impurity fraction, this effect will be explored. Beyond fueling hot edge, high confinement discharges with low recycling, these results indicate a path to fueling tokamaks and stellarators by introducing a modest lithium impurity fraction; similar results are predicted for a fusion plasma with a modest helium ash fraction. A modest lithium impurity has little effect on Z effective, and would getter common higher Z impurities present on the surfaces of metallic plasma facing such as carbon and oxygen. Results for neutral beam fueling with solid and liquid lithium plasma-facing surfaces will be presented. The dependence of beam fueling on lithium concentration and on surface conditions as monitored by in-situ secondary ion mass spectroscopy and temperature programmed desorption will be discussed.
This work is supported by USDoE contracts DE-AC02-09CH11466, DE-AC52-07NA27344.